I have now finished boxing in the Spit’s bodyshell to the chassis which will improve the chassis stiffness a lot (by an estimated 37% over standard just from the additional width added to the chassis rails, excluding the sills, roof structure and roll hoop which some seriously dodgy maths aside is pretty impressive!). However, there’s still a point in the chassis just before the front bulkhead where it’s only the chassis rails providing stiffness which just won’t do (see first terrible MS Paint doodle, which is the front section of car in profile view).
As a bit of a recap, here’s my thought process:
Most people concerned with stiffness are racers so simply extend their cage through the bulkhead to meet the suspension towers (see second crayon scribble). As I don’t want a full cage, just running a tube from the suspension towers to the bulkhead would work too. Unfortunately, in the event of a crash that would result in a weld-hardened spar of tube punching through the flimsy bulkhead and spearing my feet (see third child’s art project).
The next option was to construct an x-frame between the suspension tower and a reinforced bulkhead. Unfortunately again, without reinforcing the rest of the body structure behind it (which is tight on legroom anyway), I’m fairly certain the whole thing would just pivot backwards and crush my feet instead of spearing them :S (see fourth Tate Modern exhibition).
So, with liberal application of what is generously described as ‘brainpower’, I came up with the final Impressionist masterpiece. The lower brace (2mm wall tube) is attached to a beefed up existing body mounting bracket on the bulkhead and a point on the chassis. A thicker-wall tube is then placed from the centre of that brace to locate the suspension tower. In the event of a crash, the 2mm wall tube should bend where it meets the thicker tube, preventing any premature podiatry appointments.
The first step in this plan not to cripple myself was to make doubly damn certain that bracket on the bulkhead wasn’t going anywhere in the event of a crash. Here it is in all it’s 2mm steel glory (down at the bottom):
First step is to chip off all of the underseal gunk and make sure the steel behind it is solid.
So far so good. Then onto some Binky-esque CAD (Cardboard Aided Design) templates.
First plate made from 3mm steel was easy enough. Simple cut, bend roughly into shape, tack one end, beat with a hammer until it’s the shape it needs to be and then weld in fully.
The next one required a cup of tea and some thinking time. I could bend it roughly to shape with some muscle and a vice, but it wasn’t quite as square as I’d hoped. Luckily, a piece of spare box section I had seemed the right shape so I used the vice to press it into a better shape.
A bit more hammering and a bit more pressing and voila!
Very neat :)
Drill some big holes for some beefy plug-welds and spray with some weld-through primer in an attempt to rust-protect the back slightly...
...and tack in place.
This as proof that every now and again I can lay down a weld that looks bang-on ;)
More primer and a new plate over the top.
Tacked in place...
...hammered roughly to shape...
...some more hammering...
...and done! Except the fully welding of the bit at the bottom which I forgot to take a pic of...
Hopefully that should be a proper strong-point for the tubing to mount to, and the double-thickness of 3mm steel welded to the chassis extension should prevent it tearing off there and pivoting backwards. I’ve done the other side in an identical manner as well and one more job ticked off the list :)
Welding-wise, I’ve got one more plate tying the hardtop to the roll hoop, welding the roll hoop feet fully to the bodyshell, re-locating the rear suspension mounts inboard of the chassis for better geometry and ground clearance, and the brace structure explained at the start of this post.
Then I might be able to do something that’s not welding! God forbid!